Coordinated Universal Time

Coordinated Universal Time or UTC, also sometimes referred to as "Zulu time", is an atomic realization of Universal Time or Greenwich mean time, the astronomical basis for civil time. Time zones around the world are expressed as positive and negative offsets from UT. UTC differs by an integral number of seconds from atomic time and a fractional number of seconds from UT1.

UTC is a hybrid time scale: the rate of UTC is based on atomic frequency standards but the epoch of UTC is synchronized to remain close to astronomical UT. When the atomic second was adopted as part of the SI system of units, its rate was generally faster than the average rate of UT in the latter half of the 20th century. For this reason, UT lags behind atomic time measured by atomic clocks. UTC is maintained within 0.9 s of UT1; leap seconds are added (or, theoretically, subtracted) at the end of any UTC month as necessary. To date — the first being in 1972 — all such adjustments have been positive and applied on dates June 30 or December 31, where an additive leap second is designated as T23:59:60. The announcement of leap seconds is made by the International Earth Rotation and Reference Systems Service, based on precise astronomical forecasts of the Earth's rotation.

For most practical and legal-trade purposes, the fractional difference between UTC and UT (or, GMT) is inconsequentially small, and for this reason UTC is colloquially called GMT sometimes, even if this is not technically correct.

General information

"UTC" is not a true abbreviation; it is a variant of Universal Time, abbreviated UT, and has a modifier C (for "coordinated") appended to it just like other variants of UT. It may be regarded as a compromise between the English abbreviation "CUT" and the French abbreviation "TUC" (temps universel coordonné).

International standard UTC time can only be determined to the highest precision after the fact, as atomic time is determined by the reconciliation of the observed differences between an ensemble of atomic clocks maintained by a number of national time bureaus. This is done under the auspices of the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) (BIPM). However, local clusters of atomic clocks are sufficient for accuracy to within a few tens of nanoseconds.

Drag when the clock shows the UTC hour calculating it from your local computer clock. You can see if a UTC clock is a drag one by changing your local computer clock: if UTC hour varies, it is a drag UTC clock.

Autonomous, if it is not a drag clock. This is the best class of UTC clock.

Showing the hour:

Static: the time does not change from the latest reload.

Dynamic: the time changes from minute to minute

As indicated in the standards, it is convenient to include the UTC date too.

The UT time zone is sometimes denoted by the letter Z since the equivalent nautical time zone (GMT) has been denoted by Z since about 1950, and by a "zone description" of zero hours since 1920. See Time zone#History. Since the NATO phonetic alphabet and radio-amateur word for Z is "Zulu", UT is sometimes known as Zulu time.